Tobacco and alcohol use and genetic susceptibility are major risk factors for squamous cell carcinoma of the head and neck (SCCHN). Identification of susceptible individuals can effectively facilitate prevention of this disease by avoiding tobacco and alcohol use. Tobacco carcinogens cause a variety of DNA damage in the target cells, which may lead to uncontrolled cell growth, but the cells evolve to have the mechanism of programmed cell death (apoptosis), which helps eliminate cells with excessive DNA damage and thus reduce cancer risk. At least two known apoptotic pathways, the intrinsic and extrinsic, lead to cell death in response to excessive DNA damage, and there is an established flow-cytometry method to detect the apoptosis phenotype. In this new grant application, we propose to perform apoptosis phenotyping and genotyping assays in 600 newly recruited patients with SCCHN and 600 control subjects and to perform genotyping assays for an additional 1,000 SCCHN patients and 1,000 control subjects with stored DNA samples procured previously. A total of 434 common (including 88 putatively functional and 346 tagging) SNPs of 50 apoptosis-related genes have been selected and will be genotyped by using the SNPlex genotyping method for all 3,200 subjects (1,600 cases and 1,600 controls). Our specific aims are: AIM 1: To determine the association between 434 common SNPs (i.e., minor allele frequency e 0.05) genotypes of 50 selected apoptosis-related genes and the risk of SCCHN. We will also detect TP53 mutations and HPV infection of a subset of 480 SCCHN patients to be prospectively recruited, aiming at identifying the most susceptible subgroups in this study population. AIM 2: To determine the association between the apoptotic phenotype and the risk of SCCHN. AIM 3: To determine the functional relevance of selected common tagging SNPs in apoptotic pathways by identifying the genotypes that predict the phenotypes. We will also explore the gene-gene and gene-environment interactions using the genotyping data from all 1,600 cases and 1,600 controls and questionnaire data that characterized the smoking history of each individual and identify the most susceptible subgroups in this study population. This proposed association study is highly hypothesis driven, expanding our preliminary data on the findings of a novel p53-PHB-PIG3 apoptosis mechanism. This study will identify genetic factors that predict the apoptotic phenotype and risk of SCCHN and thus will advance our knowledge of the etiology of SCCHN. The long-term goal of this study is to identify effective biomarkers for risk assessment and to identify at-risk individuals who can be targeted for primary prevention and early detection of SCCHN in the general population.